Review of 18 publications before the widespread use of cardiac care units disclosed that the frequency of rupture of the left ventricular free wall or ventricular septum among necropsy cases of acute myocardial infarction ranged from 4 to 24% (mean 8%) (619 of 7905 cases). We analyzed the frequency of rupture of the left ventricular free wall or ventricular septum among patients studied at necropsy in our laboratory since 1968 with fatal acute myocardial infarction. Of 648 such patients, 204 (31% had rupture of the left ventricular free wall or ventricular septum. Rupture occurred in 171 (40%) of 431 patients without healed myocardial infarcts (grossly visible left ventricular scars), and in 29 (13%) of 217 patients with a healed myocardial infarct (p<.Ol). Thus, the frequency of rupture of the left ventricular free wall of ventricular septum during acute myocardial infarction appears to have increased substantially since the widespread use of coronary care units, and the frequency of rupture is nearly 3 times greater in those in whom rupture occurred during the first acute myocardial infarction compared to those with a previous infarct which healed.